This invention relates to a method for isolating a polyarylene ester from its solution in an organic solvent. More specifically, the invention relates to a method for isolating a polyarylene ester as a particulate solid having a high bulk density and a uniform particle size from a solution of the polyarylene ester in tetrahydrofuran and/or dioxane which is obtained by polycondensing dihydric phenol compound with an aromatic dicarboxylic acid or its derivative.
Polyarylene esters obtained from aromatic dicarboxylic acids or derivatives thereof such as dicarboxylic acid dichlorides and dihydric phenol compounds are polymers having good thermal stability and fire retardency and superior mechanical and other properties. These polymers are produced by various known methods such as interfacial polycondensation, solution polycondensation and melt polycondensation.
In particular, the interfacial polycondensation has generally gained acceptance as an advantageous commercial process because it gives polymers of high degrees of polymerization at low temperatures. The conventional interfacial polycondensation technique for the production of polyarylene ester utilizes the interface between water and a water-insoluble organic solvent such as dichloromethane, dichloroethane, xylene or toluene in performing the polycondensation reaction. The resulting polymer is obtained as a solution in the water-insoluble organic solvent. Isolation of the desired polymer from the reaction mixture is generally effected by a procedure which involves first separating the reaction mixture into a polymer solution layer and an aqueous layer, subjecting the polymer solution layer to a purification treatment to remove impurities present in it (such as a sodium salt of unreacted dihydric phenol compound and NaCl formed as a by-product by the reaction) to the greatest possible extent, and then separating the polymer from the purified polymer solution. The purification treatment of the polymer solution is usually carried out by repeating a procedure of washing the polymer solution with a large quantity of an alkaline or acidic aqueous solution or water and then separating the organic layer from the aqueous layer. Isolation of the polymer from the purified polymer solution is effected usually by concentrating the polymer solution to gel the polymer, or by contacting the polymer solution with a large quantity of a poor solvent for the polymer, such as acetone or methanol, to precipitate the polymer and then separating it by filtration.
Thus, in the interfacial polycondensation method in which a water-immiscible organic solvent is used, the polymer solution must be purified before isolating the polymer from it, and it is a serious disadvantage of this method. The reaction between the aromatic dicarboxylic acid dichloride and the dihydric phenol compound occurs at the interface between the organic solvent layer and the aqueous layer. After the reaction, when the organic solvent layer is separated from the aqueous layer, a small amount of the aqueous layer containing impurities remains in the organic solvent layer without being completely separated from it. When the solution of the polymer in the organic solvent is directly subjected to an operation of isolating the polymer without prior purification, the polymer will be contaminated by the impurities, and decrease in purity, and consequently, its heat resistance is not satisfactory. Accordingly, the purification treatment of the polymer solution cannot be omitted. The washing operation and the operation of separating the organic layer from the aqueous layer in this purification treatment are complicated, and it is very disadvantageous in commercial operations to repeat such a complicated procedure.
In addition to this disadvantage, the prior art interfacial polycondensation method suffers from other industrially serious defects. For example, the isolation of the polymer is complicated, or the recovery of poor solvents such as methanol and acetone, and reaction solvents such as dichloromethane and dichloroethane is inefficient.
In an attempt to remove these defects of the interfacial polycondensation method for producing polyarylene esters, an interfacial polycondensation method using an organic solvent which is partially or completely miscible with water as a reaction solvent was suggested, for example, in Japanese Laid-Open Patent Publications Nos. 21498/74 and 38388/76. According to the methods disclosed in these prior art references, a solution of an aromatic dicarboxylic acid dichloride in tetrahydrofuran or dioxane, a solution of a dihydric phenol compound in tetrahydrofuran or dioxane, and an aqueous solution containing an alkali metal hydroxide, and/or an alkaline earth metal hydroxide and an inorganic electrolyte such as sodium chloride are contacted with one another to perform a polycondensation reaction. When the reaction is over, the polymer is in the form dissolved in tetrahydrofuran or dioxane, and an emulsion consisting of this solution and an aqueous layer containing the inorganic electrolyte is formed. The emulsion is poured into a large quantity of water to precipitate the polymer, and the precipitated polymer is separated by filtration to separate it as a powder. According to this method, washing of the separated solid powder with water is sufficient for the purification of the polymer, and it is not necessary to purify the polymer while it is still in the form of a solution. Hence, this method is regarded as very advantageous over the aforesaid interfacial polycondensation method using a water-immiscible organic solvent.
This method, however, still has some defects. For example, the resulting polymer in the form of a fine particulate solid does not necessarily have a uniform particle size and a high bulk density, and it is difficult to obtain a polymer substantially free from impurities such as inorganic salts only by washing with water. In particular, since the organic solvent solution of the polymer is poured into a large amount of water, the water-miscible reaction solvent becomes a large amount of a dilute aqueous solution, and it is by no means easy to recover the reaction solvent from this dilute aqueous solution. The solvent recovering operation is very troublesome, and is also inefficient from the viewpoint of energy. This is a serious defects of this method from a commercial standpoint.
In view of the fact that while the interfacial polycondensation method using a water-miscible organic solvent such as tetrahydrofuran or dioxane has been considered as advantageous for the production of polyarylene esters, it still suffers from a serious defect in isolating the resulting polymer, the present inventors have worked extensibely, and discovered a unique phenomenon which a solution of a polyarylene ester in tetrahydrofuran and/or dioxane exhibits. This discovery finally led to the accomplishment of the present invention.
It is an object of this invention to provide a method for recovering a polyarylene ester as a solid by a simple and convenient procedure from a solution of the polyarylene ester in a water-miscible organic solvent.
Another object of the invention is to isolate a polyarylene ester as a particulate solid having a high bulk density and a uniform particle size from its solution.
Still another object of this invention is to recover the reaction solvent easily from the water-containing solution remaining after separation of the polyarylene ester, and thus to remedy the greatest defect of the method for producing polyarylene esters by interfacial polycondensation using a water-miscible organic solvent as a reaction medium.